The present invention relates to data transmission procedures, and particularly procedures involving four-phase modulation of digital signals.
In data transmission systems it is usually necessary to transmit, in addition to the useful information, certain code words of fixed content and agreed-upon meaning, for example, to defined those certain points in time which identify the beginning of a long sequence of signals or an address.
At the receiving end, these particular code words must be recognizable with the greatest possible certainty. This is made difficult due to noise which is superposed on the signals as well as superposed interference signals, mainly when transmission takes place in multiple phase modulation. With two-phase, or quadrature, modulation the identification of particular code words is more dependable.
In the system described by A. Ogawa and M. Ohkawa under the title "A New Eight-Phase Modem System for TDMA" as contribution D4 to the conference report of the Second International Conference on Digital Satellite Communication, Paris, France, November, 1972, these particular code words are transmitted in two-phase modulation. A special two-phase demodulator, which has its input connected in parallel with the input of the multiple-phase demodulator, is provided to receive these particular code words. While this arrangement seems to be unacceptably expensive, the fabrication expense for the transmitting end, however, is relatively low. One reason for this is that, with a suitable selected code word, a multi-phase modulator will furnish two-phase signals.
Codeword receivers are customary and there are known embodiments which, for every type of modulation, can indicate reception of a particular code word even if the bits in only a given high proportion of the known number of bit positions of the code word have been determined to be correct. This is called "soft" code word correlation in contrast to "hard" code word correlation in which all bit positions of the particular code word must be received correctly.
In order to solve a different problem, a "soft" evaluation of the individual binary signals has also been proposed in which the evaluator emits not only "O" and "L" (L representing binary one) signals or "-1" and "+1" signals, respectively, but also intermediate value signals in a range around the zero point. Another term for this mode of operation is "half-analog".